(1) Field of the Invention
The present invention relates to a developing device and an image forming apparatus including the developing device. In particular, the present invention relates to technology to keep the amount of developer stable in a developing chamber in a trickle-type developing device using two-component developer.
(2) Description of Related Art
As a developing device included in an electrophotographic image forming apparatus, a developing device employing a two-component development method is in widespread use. The two-component development-type developing device uses developer including toner and carrier.
As such a developing device is used for a long period of time, the more amount of dust adheres to surfaces of the carrier. This gradually reduces an ability of the carrier to charge toner (the carrier with low charging ability is referred to as spent carrier), and leads to a problem like a fog, toner dispersion, and so on.
In order to solve the above-mentioned problem, a so-called trickle-type developing device has been proposed. The trickle-type developing device discharges developer including the spent carrier little by little while supplying fresh developer little by little, and thus curbs the increase in the amount of the spent carrier.
With this structure, by utilizing an increase in the volume of developer due to addition of fresh developer, excessive developer including the spent carrier are discharged little by little and replaced by developer including fresh carrier. Therefore, the problem like a fog and toner dispersion becomes less likely to be caused.
A basic subject on the trickle-type developing device is keeping the amount of developer, more specifically the amount of carrier, stable in the developing device.
Discharging an amount of developer exceeding the amount necessary to replace carrier with low charging ability is inefficient because maintenance costs increase unnecessarily. Moreover, excessive discharge of developer is nothing but a temporary reduction in the amount (volume) of developer in a developing device, and leads to a shortage of developer. Therefore, a sufficient amount of developer is not supplied to a developing roller, resulting in negative effects like uneven development.
In order to keep the amount of developer in a trickle-type developing device stable, various efforts have been made.
As one of the efforts, the following structure has been proposed. An opening for discharging developer is provided at a portion of the developing chamber, in particular at a portion higher than an assumed surface level of developer, so that an excessive amount of developer caused by supplying fresh developer is overflowed and discharged from the opening.
However, it is commonplace for an electrophotographic image forming apparatus to change a speed in the whole process (system speed) depending on a sheet type, an image quality mode, and so on. Therefore, a speed of conveying developer in a developing device also changes depending on a selected mode. Due to a change in a circulation speed of developer, a partial change in pressure and volume of developer is caused. With the above-mentioned structure, discharging of developer is controlled only by the volume of developer, and therefore directly affected by pressure fluctuations caused by a change in a driving speed. Therefore, it is difficult to keep the amount of developer stable.
Considering the structure of a developing unit, the discharge opening for overflowing developer is commonly provided at an end portion located downstream in a developer conveyance direction (hereinafter, referred simply to as “downstream”) of a conveyance passage in which a developer conveyance screw is housed. With this structure, developer is more likely to be discharged in high-speed driving, because a pressure applied to developer increases at the discharge opening. On the other hand, developer is less likely to be discharged in low-speed driving, because the pressure applied to developer decreases at the discharge opening.
This causes a large difference in a stable amount of developer in a developing chamber between high-speed driving and low-speed driving. In particular, a developing chamber has a shortage of developer while the device is driven at a high speed, or when the device is driven at a low speed after being driven at a high speed. This increases the risk of uneven development caused due to short supply of developer.
In order to solve the above-mentioned problem in a developing device utilizing an overflow of developer from a discharge opening provided at a high portion of the developing chamber, a developing device that utilizes not only a change in the volume of developer but also pressure balance in conveying developer is proposed.
Such a pressure balanced developing device controls the amount of developer to be discharged not by providing a discharge opening at a high portion of the developing chamber, but by providing a conveyance member, in front of a discharge opening, that is capable of conveying developer in a direction opposite to the developer conveyance direction. A pressure is applied by the conveyance member in the direction opposite to the developer conveyance direction. The amount of developer to be discharged is controlled by utilizing balance between a pressure applied in the developer conveyance direction and the pressure applied in the opposite direction.
Specifically, the following structure is commonly used. At a downstream end of a normal conveyance screw that does not face a developing roller (hereinafter, ends of a screw not facing a developing roller are also referred to as “hidden portions of a screw”), another screw helically winding in a reverse direction is formed. In addition, a discharge opening is provided further downstream in a conveyance passage than the reverse screw. The discharge opening is provided at a position lower than a normal surface level of developer, and therefore does not have an ability to control the amount of developer to be discharged. Developer that overcomes a pressure applied by the reverse screw in the direction opposite to the developer conveyance direction is conveyed and discharged from the discharge opening.
An advantage of the structure is that the amount of discharge developer is controlled by utilizing not only a change in the volume of developer but also pressure balance in conveying developer. In particular, when the developing device is driven at a high speed or also is driven at a low speed, the pressure applied in the normal developer conveyance direction and the pressure applied in the opposite direction change simultaneously. As a result, the amount of discharged developer is kept relatively constant by the pressure balance therebetween.
However, the above-mentioned pressure balanced developing device does not completely eliminate a difference in the amount of developer in a developing chamber between high-speed driving and low-speed driving.
The reason is as follows. In high speed driving, a conveyance force acting in a forward direction that is applied by the normal conveyance screw is increased, and also a conveyance force acting in a reverse direction that is applied by the reverse screw is increased. However, the conveyance force in the forward direction is normally set to be greater than the conveyance force in the reverse direction. Therefore, the conveyance force in the forward direction increases more greatly than the conveyance force in the reverse direction, and thus a difference therebetween becomes greater. As a result, developer to which a high pressure is applied overcomes the reverse screw, and is discharged unnecessarily. A larger amount of developer can be discharged in high speed driving than that in low speed driving.
In particular, in order to respond to a recent demand for miniaturization of an image forming apparatus, the developing device itself needs to be made small in size, and thus a developing chamber in a developing device needs to be made small in capacity. Accordingly, even a small change in the amount of discharged developer significantly affects a surface level of developer in a conveyance passage facing a developing roller. When the surface level of developer is lowered due to temporary discharge of an excessive amount of developer, a sufficient amount of developer cannot be supplied to the developing roller, resulting in occurrence of uneven development.